Supposebly

A short rant:
Why is there a vast number of people here, where I work that say "supposebly"? Do they not realize that it is a d and not a b? I cringe every time I hear it; it irritates me to no end. I bite my tongue each time so I don't correct them. Perhaps I shouldn't.
So, in my rant I think maybe I'm wrong and supposebly is a word. So I looked it up on dictionary.com and supposably is a word. It is defined as an adverb meaning capable of being supposed. Supposedly (adverb) is defined as believed or reputed to be the case. So now, I'm humbled and confused.